Car top rack systems, or roof rack arrangements, can be of the static type, having no movable parts, or of the slideable type, having an extendable load carrying surface. An extendable roof rack arrangement is disclosed in the published patent application WO 2005/44618, which discloses an extendable cross bar system. The extendable cross bar system comprises a stationary portion and an extendable portion. A sliding low friction pad is arranged between the stationary portion and the extendable portion to reduce the friction between these portions.
Another vehicle roof rack system is disclosed in the patent publication AU 2003231667 A1. This document discloses a solution which incorporates an inner and outer member, the inner member being slideable to an extended position. The vehicle roof rack system comprises a special end section to provide a neat end cap for the aperture formed by the outer member. Both these documents are silent about the problems which arise when using slideable members in roof rack systems. The documents are further silent about the problems which can arise from the intrinsic forces imparted to the roof rack system e.g. from the load carrier feet.
Generally there are two different types of load carrier feet used in roof rack systems; clamp rack feet (including rack feets for rails) and fix point feet. A load carrier foot which uses a clamping force imparted between a support surface and a gripping member as primary attachment force is generally of the clamp rack type, and a load carrier foot which are cooperating with pre prepared positions on the vehicle, is generally of a fix point foot.
During attachment of a clamp rack load carrier foot to the roof of the vehicle, the clamp rack load carrier foot rests on the roof and a grip member engages a ledge on the vehicle. The grip member is thereafter forced in an upwardly by means of attachment means, such as a screw. The attachment means imparts a clamping force between the support surface and the grip member which retains the clamp rack load carrier foot to the roof of the vehicle. A problem with a clamp rack load carrier foot of this type is that this force also imparts the clamp rack load carrier foot with a force component which acts to rotate the clamp rack load carrier foot towards the roof of the vehicle, i.e. each clamp rack load carrier foot tend to force themselves towards each other during attachment. As a consequence, intrinsic forces are formed within the load carrying bar. This could manifest itself by that the stationary part of the roof rack system bends, i.e. forms an arc shape along its length. Such intrinsic forces severely affect the performance of an extendable roof rack system as the extendable part can stick on the arc shaped stationary part. Intrinsic forces imparted via the clamp rack load carrier foot could also arise from misaligned load carrier feet or a wrongly positioned or assembled load carrier foot.